Petrified Forest National Park
June 9, 2012 2 Comments
The record of petrified wood, across all continents, and back to the Devonian period tells about the history of earth’s forests. Petrified wood preserves information about things like taxonomic identity, canopy structure, seasonality, and productivity. Coupled with an understanding of the sedimentogical context of the rocks in which the wood is preserved we can learn even more, like the soil preferences and spatial structure of the trees (if they are preserved in place) or about the events that resulted in the trees’ burial and preservation.
I was recently on a trip with my wife that took us through Petrified Forest National Park during National Park week (not by accident) where the Triassic Chinle Formation is exposed and the remains of a ancient forests are preserved. I’d never been before, but I’d seen the wood in museums, and am familiar with some of the vertebrate paleontology that is going on involving some Smithsonian scientists; so I was excited to finally experience the place.
Obviously, I expected to see a lot of petrified wood, but even so I was impressed! Vast fields of deep red silicified wood dot the landscape not far from historic route 66 which runs through the park. Walking trails wind through some of the fields, taking the visitor past impressive specimens, while in other areas visitors can look down into valleys littered with wood. As erosion exposes the grey-blue and red sediment of the ancient floodplain deposits (which make for beautiful backdrops), the wood-bearing horizons are occasionally exposed. When this happens the mud and sand washes away, but the heavy petrified wood rests on the surface.
The wood comes from several levels in the Chinle Formation, but most of it comes from the Petrified Forest Member and the Sonsela Member. These units are a sequence of channel fills and floodplain deposits which no doubt supported vegetation, but the trees that produced the wood known as Araucarioxylon arazonicum, (Triassic conifer wood) were not preserved in growth position. None of the logs are upright and none of them have roots, though the largest specimens show the basal flare. The wood is generally found in the cross-bedded conglomerates and cross-bedded conglomeratic sandstones. These are the channel fills, which rest on scour surfaces with 1-7 meters of relief. This all suggests that ~210 Million years ago there was high energy flow capable of transporting and then burying the tremendous logs, sometimes forming log jams.
The longest petrified logs measured at the park are ~43 meters (~140 feet) and up to 3 meters in diameter. The canopy may have been 60 meters (~200 feet) high. More than ten other types of petrified wood have been identified, but most of them are rare. In addition to conifers, there are also tree-ferns, a Ginkgo relative, and Calamites. The wood doesn’t have annual growth rings, which tells us that growing conditions were generally good year-round; a conclusion that is consistent with the presence of tree ferns which generally don’t tolerate cold temperatures.
If you get a chance to go through Petrified Forest, I highly recommend it. We had a great time and they do a good job of painting the picture of the ancient ecosystem, including the early dinosaurs and other animals.